That's because you didn't read the footnotes to part 2
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Footnotes
*1 The most salient noise sources upstream of ISO amplification include
reset noise, pixel amplifier noise, and dark current shot noise for
longer exposures (which we won't dive into). Reset noise originates from
variations in the voltages each pixel is reset to after a charge is read
(ideally, they'd all be reset to the exact, same voltage). These
variations - that show up as changes in intensity across neighboring
pixels - can largely be mitigated by a process called correlated double
sampling (CDS), which samples and subtracts the reset voltage at any
pixel from the total voltage due to exposure. It can almost be thought
of as a dark frame subtraction, and is very effective, lowering
pixel-to-pixel variations, as well as pixel-level amplifier noise, to
the level of single electrons in modern sensors. However, CDS isn't
completely effective, and so a non-zero noise component still remains.
For reasonable exposure lengths, any remaining reset and pixel amplifier
noise post-CDS are probably the largest sources of upstream read noise.
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*2 Downstream read noise includes all sources from and after, or
'downstream of,' ISO amplification. This includes noise from the
programmable gain amplifier (set by your ISO setting), noise from the
analog to digital conversion process, as well as any noise introduced in
the pathways between all these electronic components in your camera.
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*3 How much of this light a camera is able to make use of depends on its
sensor efficiency, which is beyond the scope of this article. We mention
it here, though, because sensors with higher efficiencies will record
more of the available light during any given exposure: leading to less
shot noise and a slightly cleaner result in our exposure latitude test.
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*4 There are other sources of noise that are beyond the scope of this
article. Thermal noise builds up with long exposures, amplifier glow can
lead to 'hot spots' at the edges of your image, pixel-response
non-uniformity can lead to noise even in brighter areas of your image,
and quantization error further limits signal-to-noise ratio in shadows.
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Chuck Norcutt
On 9/22/2015 8:47 AM, Wayne Harridge wrote:
Thanks (I think) for that reference Moose.
Unfortunately I don't think it addresses increased noise from long
exposures, ok the relative shot noise increases when the amount of light
hitting the sensor is low and you tend to use longer exposures when the
light level is low, but this is also the time when you also might opt for
high ISO. No mention of increased noise at long exposures due to heating of
the sensor.
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